Heretofore, nitride semiconductor devices have been used in blue LEDs (light emitting diodes) serving as light sources for lightings, backlights, and the like, and in green LEDs serving as light sources for white lightings. Further, in a white lighting, white color is formed by using a red LED, a blue LED, and a green LED.
As shown in FIG. 11, in the nitride semiconductor device, a nitride semiconductor layer formed of an n-type semiconductor layer, an MQW active layer 505, and a p-type semiconductor layer are laminated on a substrate 500. It is preferable that the substrate 500 has conductive properties so that the substrate 500 can form a path for electric current to flow into the nitride semiconductor layer via the substrate 500. However, a substrate having conductive properties costs more as compared to a substrate not having conductive properties. For this reason, there has been considered an attempt to impart the conductive properties by doping a dopant to a substrate not having the conductive properties.
However, when the substrate 500 is doped with the dopant, a light absorption amount at the substrate 500 increases, and the characteristics of the nitride semiconductor device are thereby affected. Accordingly, a substrate not having conductive properties and not doped with any dopant is generally used (for example, a sapphire substrate or the like) as the substrate 500.
When a substrate not having the conductive properties and not doped with any dopant is used as the substrate 500, the nitride semiconductor device needs to have a path through which electric current flows, on one of a main surface side of the substrate 500.
Specifically, as shown in FIG. 11, the nitride semiconductor layer (an n-type buffer layer 501, an n-type contact layer 502, an n-type clad layer 504, an MOW active layer 505, a p-type clad layer 506, and a p-type contact layer 507) is laminated on the substrate 500, and then a part of the nitride semiconductor layer is etched from the p-type contact layer 507 side, until an n-type contact layer 502 is exposed. Next, an n-electrode 514 is formed on a main surface of the exposed n-type contact layer 502. Further, a p-electrode 508 is formed on a main surface of the p-type contact layer 507. Thereby a nitride semiconductor device, which contains a path between the n-electrode 514 and the p-electrode 508 through which the electric current flows without passing through the substrate 500 there between, can be formed.
However, it is a characteristic of the electric current that the electric current flows a shorter path under the same resistance value. Therefore, in the conventional nitride semiconductor device as shown in FIG. 12, the electric current is concentrated in a portion corresponding to a line L from the p-electrode 508 to the n-electrode 514 when electric current is caused to flow between the n-electrode 514 and the p-electrode 508. This leads to a problem that the electric current does not flow evenly throughout each layer of the nitride semiconductor device.
As a result, it has been difficult to evenly emit the light from the MQW active layer 505 of the nitride semiconductor device.
Moreover, according to the above-described nitride semiconductor device, since an electric voltage is concentrated in the portion corresponding to the line L from the p-electrode 508 to the n-electrode 514 in the same manner as the flow of the electric current. This leads to a problem that an electrostatic breakdown tends to occur in the corresponding portion.
In addition, according to the above-described nitride semiconductor device, since the p-electrode 508 and the n-electrode 514 are formed on one of a main surface side of the substrate 500, a problem arises that a chip area required for the nitride semiconductor device increases as compared to a nitride semiconductor device having an n-electrode formed on one of the main surface side of a substrate and a p-electrode formed on the other main surface side thereof. Accordingly, a productivity of the nitride semiconductor decreases.
In order to solve the above problems, a method of manufacturing a counter electrode type nitride semiconductor device has been proposed. In this proposed method, a nitride semiconductor layer is laminated on a substrate, and a p-electrode is formed on one of the main surface of the nitride semiconductor layer, the nitride semiconductor layer is then separated from the substrate, and an n-electrode is formed on a main surface on the opposite side of the surface on which the p-electrode of the nitride semiconductor layer is formed.
Specifically, in an example of a counter electrode type nitride semiconductor device using a substrate made of sapphire and a nitride semiconductor layer made of a GaN-based semiconductor, the nitride semiconductor layer is laminated on the substrate, and a p-electrode is formed on a main surface of the nitride semiconductor layer.
Next, the nitride semiconductor layer side is irradiated from the substrate side with excimer laser light having a wavelength of approximately 300 nm or less with irradiation energy of several hundred mJ/cm2. Thereby, the nitride semiconductor layer is thermally decomposed near a boundary surface of the substrate and the nitride semiconductor layer, to separate the nitride semiconductor layer from the substrate, thereby forming an n-electrode on an exposed main surface of the nitride semiconductor layer.
The method of manufacturing a nitride semiconductor device for obtaining the counter electrode type nitride semiconductor device in this manner has been disclosed (for example, see Japanese Patent Application Publication No. 2003-168820).
Using the above manufacturing method makes it possible to obtain a counter electrode type nitride semiconductor device, which forms a p-electrode on one of the main surface of a nitride semiconductor layer and an n-electrode is formed on the other main surface thereof, in a same manner as a substrate having the conductive properties. Therefore, a nitride semiconductor device having an improved light extraction efficiency can be manufactured.